Thin ferroelectric film element having a multi-layered thin ferroelectric film and method for manufacturing the same

ABSTRACT

A thin ferroelectric film element comprises upper and lower thin electrode films and a thin ferroelectric film formed on a substrate, wherein the thin ferroelectric film comprises at least three layers in which at least one layer has a composition of constituent elements different from those of the other layers and a resistivity higher than that of the other layers, and at least two layers of the others are the same composition of constituent element.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thin ferroelectric film element andto a method for manufacturing the same. The thin ferroelectric filmelement of the present invention can be used for a memory element (e.g.,a capacitor), a pyroelectric sensor (e.g., an infrared linear arraysensor, a supersonic sensor), and a piezoelectric element (e.g., anoptical modulator).

2. Description of the Related Arts

Thin ferroelectric films have numerous functions such as spontaneouspolarization, high dielectric constant, electrooptical effect,piezoelectric effect and pyroelectric effect, and hence are applied to awide range of device developments. For example, thin ferroelectric filmsare applied to infrared linear array sensors by utilizing theirpyroelectricity, to supersonic sensors by utilizing theirpiezoelectricity, to optical modulators of a waveguide type by utilizingtheir electro-optical effect, and to capacitors for DRAMs and MMICs byutilizing their high dielectric constant.

Especially among these various application devices, there has been adevelopment of ferroelectric non-volatile memories (FRAMs) that arehighly dense and can operate at a high speed by combination with asemiconductor memory technique in accordance with the recent progress ofthin film formation technique. Non-volatile memories incorporating athin ferroelectric film are under extensive research and development forpractical use not only as a replacement for conventional non-volatilememories but also as a memory that can be substituted for SRAMs andDRAMs owing to their properties such as high speed writing/reading,low-voltage operation, and high endurability in writing/reading.

In conducting these device developments, a material is required that hasa large residual polarization (Pr), low coercive field (Ec), low leakagecurrent, and endurability in repetition of polarization inversion.Further, it is preferable that the above properties are achieved with athin film of 2000 Å or less so as to decrease the operation voltage andto conform to the fine processing of semiconductors.

Here, oxide materials having a perovskite structure such as PZT (leadtitanate zirconate, Pb(Ti_(x) Zr_(1-x))O₃) have been mainly used asferroelectric materials for these purposes. However, in the case of amaterial such as PZT containing lead as its constituent element, leadtends to evaporate at the time of forming a film due to high vaporpressure of lead or its oxide, whereby defects or, in extreme cases,pinholes are generated in the formed film. This may increase the leakagecurrent and, when the polarization inversion is repeated, this may causea fatigue phenomenon in which the spontaneous polarization decreases.Particularly, in view of substituting ferroelectric non-volatilememories for FRAMs, it must be ensured, with respect to the fatiguephenomenon, that the characteristics remain unchanged even after 10¹⁵times repetition of polarization inversion. Accordingly, the developmentof a thin ferroelectric film without a fatigue has been desired.

Meanwhile, a research and development of bismuth layered-structurecompound materials has been recently taking place as a ferroelectricmaterial for FRAMs. The bismuth layered-structure compound materialswere found by Smolenskii and others in 1959 (G. A. Smolenskii, V. A.Isupov and A. I. Agranovskaya, Soviet Phys. Solid State, 1, 149(1959)),and were subsequently examined in detail by Subbarao (E. C. Subbarao, J.Phys. Chem. Solids, 23, 665(1962)). Recently, Carlos A. Paz de Araujoand others have found that thin films of bismuth layered-structurecompound are suitable for application to integrated circuits offerroelectrics and high dielectrics, and have reported an excellentfatigue property that the characteristics remain unchanged even after10¹² times repetition or more of polarization inversion (InternationalApplication No. PCT/US92/10542).

The bismuth layered-structure compound is selected from a compound ofthe formula Bi₂ A_(m-1) B_(m) O_(3m+3) (wherein A is selected from Na,K, Pb, Ca, Sr, Ba and Bi; and B is selected from Fe(III), Ti, Nb, Ta, Wand Mo; and m is a positive integer). The crystal structure of thebismuth layered-structure compound is such that the (Bi₂ O₂)²⁺ layer andthe (A_(m-1) B_(m) O_(3m+1))²⁻ layer are alternately stacked. In otherwords, the basic crystal structure of the compound is such that thelayered perovskite layer having a series of perovskite lattices of(m-1)ABO₃ is sandwiched from above and below by (Bi₂ O₂)²⁺ layers. Here,it is not always the case that the elements A and B to be selected aresingle elements.

Examples of such bismuth layered-structure compound materials includeSrBi₂ Ta₂ O₉, SrBi₂ Nb₂ O₉, Bi₄ Ti₃ O₁₂, BaBi₂ Nb₂ O₉, BaBi₂ Ta₂ O₉,PbBi₂ Nb₂ O₉, PbBi₂ Ta₂ O₉, SrBi₄ Ti₄ O₁₅, PbBi₄ Ti₄ O₁₅, Na₀.5 Bi₄.5Ti₄ O₁₅, K₀.5 Bi₄.5 Ti₄ O₁₅, Sr₂ Bi₄ Ti₅ O₁₈, Ba₂ Bi₂ Ta₅ O₁₈, Pb₂ Bi₄Ti₅ O₁₈ and the like.

The method for manufacturing a thin ferroelectric film may be a physicalmethod such as vacuum vapor deposition method, sputtering method andlaser abrasion method, or a chemical method such as sol-gel method, MOD(Metal Organic Decomposition) method or MOCVD (Metal Organic ChemicalVapor Deposition) method employing thermal decomposition and oxidationof an organic metal compound used as a starting material to produceoxide ferroelectrics.

Among the above-mentioned methods for forming a ferroelectric film, theMOCVD method provides an excellent step-coverage and also may possiblybe used for low temperature film formation, so that the MOCVD method ispromising in view of manufacturing highly integrated FRAMs and hasrecently been under active research and development.

On the other hand, the sol-gel method or the MOD method has been widelyused owing to the fact that a uniform mixture in an atomic level can beobtained, that the composition can be controlled easily and thereproducibility is excellent, that no special vacuum apparatus isrequired and a film having a large area can be formed under ordinarypressure and that the industrial cost is small.

Especially, the MOD method is used for forming the above-mentioned thinfilm of bismuth layered-structure compound, and a thin ferroelectricfilm or a thin dielectric film is manufactured through the followingsteps in the film formation process according to conventional MODmethods (International Application No. PCT/US92/10542, PCT/US93/10021).

(1) Step of applying a precursor solution containing a compositealkoxide and the like onto a substrate by spin coating method or thelike for forming a film;

(2) Step of annealing and drying the obtained film at 150° C. for 30seconds to several minutes for removing, from the film, the solvent andthe alcohol and residual water that have been generated by the reactionof step (1);

(3) Step of annealing the film at 725° C. for 30 seconds under oxygenatmosphere by employing a RTA (Rapid Thermal Annealing) method forremoving the organic components in the film by thermal decomposition;and

(4) Step of annealing the film at 800° C. for one hour under oxygenatmosphere for crystallization of the film;

(5) Step of annealing the film at 800° C. for 30 minutes under oxygenatmosphere after an upper electrode is formed.

Here, in order for obtaining the desired film thickness, the steps of(1) to (3) are repeated and, finally, the steps of (4) and (5) arecarried out.

The thin ferroelectric film or thin dielectric film is thus fabricated.

However, by a method of manufacturing a thin ferroelectric film usingthe above-mentioned conventional MOD method, little crystallization ofthe thin ferroelectric film takes place at annealing temperature of 650°C. or less. Accordingly, in order to obtain a high residualpolarization, it is necessary to carry out an annealing step at anextremely high temperature of 800° C. for a period of time as long asone hour (International Application No. PCT/US93/10021). Therefore, informing a thin ferroelectric film element on an integrated circuithaving a stack structure, there will occur damages such as poor contactand deterioration in characteristics due to interdiffusion and oxidationbetween the viahole (contact hole) material and the electrode material,thus placing a hindrance particularly in manufacturing such highlyintegrated devices.

Also, since the annealing temperature is thus high, the particlediameter of the crystal particles constituting the thin ferroelectricfilm is as large as 1000 to 2000 Å and the irregularity on the surfaceof the thin film is large. Accordingly, it has not been possible toapply the conventional MOD method to fine submicron processing which isrequired in manufacturing highly integrated devices.

Moreover, in the case of highly integrated FRAMs of 4M bit to 16M bit ormore, the capacitor area will be small and the spontaneous residualpolarization Pr required in the ferroelectric materials will be large,so that Pr of at least 10 μC/cm² will be necessary. In the case of thethin SrBi₂ Ta₂ O₉ film, the spontaneous residual polarization Pr will besmall in accordance with the decrease in the annealing temperature, sothat it has not been possible to obtain sufficient Pr required in highlyintegrated FRAMs by conventional methods if the annealing temperature islowered.

On the other hand, it is known in the art that Nb is added so as toincrease Pr of the thin SrBi₂ Ta₂ O₉ film. However, if Nb is added intothe thin SrBi₂ Ta₂ O₉ film, the coercive field Ec will be large althoughPr will certainly be large. Accordingly, the leakage current willincrease in addition to the rise in the operation voltage and, moreover,the fatigue characteristics will be deteriorated.

SUMMARY OF THE INVENTION

The present invention provides a thin ferroelectric film elementcomprising upper and lower thin electrode films and a thin ferroelectricfilm formed on a substrate, wherein the thin ferroelectric filmcomprises at least three layers in which at least one layer has acomposition of constituent elements different from those of the otherlayers and a resistivity higher than that of the other layers, and atleast two layers of the others are the same composition of constituentelements.

The present invention also provides a method of manufacturing theabove-mentioned thin ferroelectric film element comprising: forming thelower thin electrode film on the substrate; applying each of a pluralityof precursor solutions containing partially different metal elements anddrying to form a laminate film of at least three layers comprising atleast two kinds of films; performing a first annealing; forming theupper thin electrode film; and performing a second annealing.

Thus, the present invention provides a thin ferroelectric film elementin which a sufficiently high spontaneous residual polarization and asufficiently low coercive field are achieved at an annealing temperaturelower than that by conventional methods, the thin ferroelectric filmelement being applicable to highly integrated FRAMs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an essential part ofa thin ferroelectric film element according to Example 1 of the presentinvention.

FIG. 2 is a diagram showing the steps of synthesizing a precursorsolution (a) to be used for manufacturing the Example 1 of the presentinvention.

FIG. 3 is a diagram showing the steps of synthesizing a precursorsolution (b) to be used for manufacturing the Example 1 of the presentinvention.

FIG. 4 is a diagram showing the manufacturing steps according to Example1 of the present invention.

FIG. 5 is a view showing a Sawyer Tower Bridge used for the measurementof ferroelectric properties in Example 1, Comparison Example 1 andComparison Example 2.

FIG. 6 is a view showing how the residual spontaneous polarization Prdepends on the applied voltage in Example 1, Comparison Example 1 andComparison Example 2.

FIG. 7 is a view showing how the coercive field Ec depends on theapplied voltage in Example 1, Comparison Example 1 and ComparisonExample 2.

FIG. 8 is a view showing how the switching electric charge ΔQ depends onthe applied voltage in Example 1, Comparison Example 1 and ComparisonExample 2.

FIG. 9 is a view showing the fatigue characteristics in Example 1,Comparison Example 1 and Comparison Example 2.

FIGS. 10(a) to 10(h) are conceptual views showing film structures ofthin ferroelectric film elements according to Examples 2 to 6 of thepresent invention.

FIG. 11 is a schematic view showing an essential part of a non-volatilememory having a capacitor structure according to Example 8 of thepresent invention.

FIG. 12 is a view showing an equivalent circuit of the non-volatilememory of FIG. 11.

FIG. 13 is a schematic view showing an essential part of MFMIS-FETaccording to Example 9 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The substrate to be used in the present invention is not specificallylimited as long as it is a substrate (or wafer) on which a ferroelectricelement, a semiconductor device including an integrated circuit and thelike is generally formed. Preferable examples of the substrate include asemiconductor substrate such as silicon and a compound semiconductorsubstrate such as GaAs.

The thin ferroelectric film element of the present invention comprisesupper and lower thin electrode films and a thin ferroelectric filmformed on the substrate. The lower electrode film to be used in thepresent invention is not specifically limited as long as it is formed ofan electrically conductive material that can be generally used as anelectrode. Preferable examples of the lower electrode film include amonolayer film of a high melting-point metal such as Ta, Ti, W or anitride thereof, Al, Cu, Pt, or an electrically conductive oxide such asRuO₂ or IrO₂, and a laminate film of these layers. The thickness of thelower thin electrode film to be formed may be, for example, about 500 Åto 2000 Å. The lower thin electrode film may be formed directly on asubstrate or indirectly on a substrate on which a desired element or acircuit has been formed and further an interlayer insulating film or thelike has been formed. However, the lower thin electrode film ispreferably formed on the substrate through the intermediary of aninsulating film such as SiO₂ or SiN.

The thin ferroelectric film to be formed on the thin electrode film ispreferably formed of at least three layers. Among these layers, at leastone layer has a different composition of constituent elements from thatof the other layers and has a higher resistivity than other layers.Here, the term "different composition of constituent elements" includesboth of the case in which the constituent elements of one layer aredifferent from those of another layer and the case in which theconstituent elements of one layer are the same as those of another layerbut the composition ratio thereof is different. The thin ferroelectricfilm is preferably formed of a bismuth layered compound containing Sr,Bi, Ti, Ta and Nb having a high electropositivity such as SBT (SrBi₂ Ta₂O₉), SBTN (SrBi₂ Ta₀.8 Nb₁.2 O₉), BTO (Bi₄ Ti₃ O₁₂), SrBi₄ Ti₄ O₁₅,SrBi₄ (Ti, Zr)₄ O₁₅, SrBi₂ Nb₂ O₉, CaBi₂ Ta₂ O₉, BaBi₂ Ta₂ O₉, BaBi₂ Nb₂O₉, PbBi₂ Ta₂ O₉, and the like, and further the above-mentioned bismuthlayered-structure compound material. The material for at least one layerhaving the different composition of constituent elements from that ofthe other layers may be selected from the above compounds as long as thematerial has a resistivity higher than the resistivity of the otherlayers. The resistivity of at least one layer is preferably about 10¹²Ω.cm or more, and the resistivity of the entire thin ferroelectric filmis preferably within the range of about 10¹³ to about 10¹² Ω.cm. It ispreferable that each of the layers constituting the thin ferroelectricfilm has a thickness of about 100 Å to about 1000 Å. A layer having athickness below about 100 Å will be stripe-like, making it difficult toform the layer. A layer having a thickness over about 1000 Å willgenerate cracks within the layer. The thickness of each layer may be thesame or may be varied.

The thin ferroelectric film element according to the present inventionpreferably has a capacitor structure in which the upper thin electrodefilm similar to the above-mentioned lower thin electrode film is formedon the thin ferroelectric film. The shape and the size of the upper andlower electrodes to be formed may be suitably adjusted in accordancewith the intended use or the like. The thin ferroelectric film elementhaving such a structure may be applied to all kinds of semiconductordevices constituting the integrated circuits, for example, MFMIS-FETs,pyroelectric elements, ferroelectric cold cathode elements and the like.

The thin ferroelectric film according to the present invention may beformed by a physical method such as vacuum vapor deposition method,sputtering method and laser abrasion method, or by a chemical methodsuch as sol-gel method, MOD method and MOCVD method using an organicmetal compound as a starting material. Among these, a preferable methodis a sol-gel method involving a series of steps including applying eachof a plurality of precursor solutions containing partially differentmetal elements and drying to form a thin ferroelectric film of at leastthree layers comprising at least two kinds of thin ferroelectric films,performing a first annealing step, forming an upper thin electrode film,and performing a second annealing step.

More specifically described, preparation of precursor solutions for thethin ferroelectric film include: dissolving a starting material such asan alkoxide or a salt of each of the metals constituting the thinferroelectric film into a suitable solvent, for example, an organicsolvent such as hexane and 2-ethylhexanate; optionally heating to atemperature of about 100° to 120° C. for promoting the reaction;stirring the solution for about 1 to 60 minutes for mixing; andoptionally heating and stirring the solution at a temperature of about130° to 150° C. The obtained precursor solutions are preferablysubjected to removal of the solvent, filtration and the like. The periodof time for stirring or reacting the solutions or precursor solutionsmay be adjusted within the range of about 1 minute to 30 hours. Each ofthe precursor solutions thus synthesized is applied onto a substrate onwhich a lower thin electrode film has been formed. The precursorsolutions may be applied by a conventional method such as spin coatingmethod or the like. The thickness and the composition of each layerconstituting the thin ferroelectric film can be controlled by adjustingthe kind, the concentration, the viscosity and the like of the precursorsolutions.

The applied precursor solutions are then dried. The drying is performedpreferably through more than two steps. The first drying step may becarried out at a temperature of about 100° to about 130° C., preferablyaround 120° C., so as to ensure uniform drying. Drying at a temperatureof more than 150° C. generates cracks by a film stress at the laminatingstep. The second drying step is preferably carried out at a temperatureabove the boiling point of the solvent, for example, about 250° to about300° C.

The above steps are repeated more than three times to form a thinferroelectric film of more than three layers.

Subsequently, a first annealing step is preferably performed after thethin ferroelectric film has been formed. The first annealing step ispreferably carried out under an oxygen atmosphere or in an air at atemperature of about 500° to about 600° C. for about 1 to 60 minutes.The first annealing step may be carried out by employing a conventionalmethod such as RTA method or heat treatment using a thermal processingfurnace.

Then, the upper thin electrode film is formed. The upper thin electrodefilm may be formed of the above-mentioned electrically conductivematerial by employing a conventional method such as vacuum vapordeposition method, sputtering method, EB vapor deposition method or thelike. A second annealing step is preferably performed after the upperthin electrode film has been formed. The second annealing step ispreferably carried out under an oxygen atmosphere or in an air at atemperature of about 600° to 800° C. for about 1 to 60 minutes. Thesecond annealing step may be carried out by employing a conventionalmethod such as RTA method or heat treatment using a thermal processingfurnace. By these annealing steps, it is possible to form a thinferroelectric film which is dense and has good surface flatness.

EXAMPLES

The preferred embodiments of the present invention will hereafter bedetailed in conjunction with the attached drawings. However, theseembodiments are not to be construed as being intended to limit the scopeof the present invention.

Example 1

FIG. 1 is a schematic cross-sectional view showing an essential part ofa thin ferroelectric film element according to Example 1 of the presentinvention. Referring to FIG. 1, the thin ferroelectric film element hasa capacitor structure including, in this order, a silicon single crystalsubstrate 1, a silicon thermal oxide film 2 of 200 nm thickness formedthereon, a Ta film 3 of 20 nm thickness formed thereon, a Pt film 4 of200 nm thickness formed thereon, a thin ferroelectric film 5 of 200 nmthickness formed thereon, and an upper Pt electrode 6 of 100 nmthickness formed thereon. The thin ferroelectric film 5 includes a SrBi₂Ta₂ O₉ layer 5a (hereafter referred to as SBT layer), a SrBi₂ Ta₀.8Nb₁.2 O₉ layer 5b (hereafter referred to as SBTN layer), and a SrBi₂ Ta₂O₉ layer 5c (hereafter referred to as SBT layer) formed in this order.

Next, a method of manufacturing the thin ferroelectric film elementshown in FIG. 1 will be explained.

First, a silicon thermal oxide film 2 is formed to a thickness of 200 nmon a (100) surface of the silicon single crystal substrate 1 by thermaloxidation of the surface of the silicon substrate 1 at 1000° C. A Tafilm 3 is formed to a thickness of 20 nm on the silicon thermal oxidefilm 2 by sputtering method. A Pt film 4 is then formed to a thicknessof 200 nm thereon. The obtained substrate is used as the substrate forforming the thin ferroelectric film.

Precursor solutions to be used for forming the thin ferroelectric film 5on the substrate were synthesized for sol-gel method in accordance withthe process diagrams shown in FIGS. 2 and 3.

First, the synthesis of the precursor solution (a) to be used forforming the SBT layers 5a, 5c will be explained. Referring to FIG. 2,tantalum ethoxide was weighed (step S1) and dissolved in 2-ethylhexanate(step S2). In order to promote the reaction, the solution was stirredwhile being heated from 100° C. up to the maximum temperature of 120°C., and the reaction was continued for 30 minutes (step S3).Subsequently, ethanol and water generated in the reaction were removedat 120° C. Into the solution was added strontium-2-ethylhexanatedissolved in 20 ml to 30 ml of xylene (step S4), and the solution washeated and stirred from 125° C. up to the maximum temperature of 140° C.for 30 minutes (step S5). Into the solution was then addedbismuth-2-ethylhexanate dissolved in 10 ml of xylene (step S6), and thesolution was heated and stirred from 130° C. up to the maximumtemperature of 150° C. for 10 hours (step S7).

The solution was then distilled at a temperature of 130° C. to 150° C.for 5 hours so as to remove the low molecular-weight alcohol, water andxylene used as a solvent from the solution. The solution was thenfiltrated with a filter of 0.45 μm diameter so as to remove dusts fromthe solution (step S8). Subsequently, the concentration of SrBi₂ Ta₂ O₉in the solution was adjusted to 0.1 mol/l to produce the precursorsolution (a) (step S9).

Next, the synthesis of the precursor solution (b) to be used for formingthe SBTN layer 5b will be explained. Referring to FIG. 3, tantalumethoxide and niobium ethoxide were weighed (steps S11a and S11b) anddissolved in 2-ethylhexanate (step S12). In order to promote thereaction, the solution was stirred while being heated from 100° C. up tothe maximum temperature of 120° C., and the reaction was continued for30 minutes (step S13). Subsequently, ethanol and water generated by thereaction were removed at 120° C. Into the solution was addedstrontium-2-ethylhexanate dissolved in 20 ml to 30 ml of xylene (stepS14), and the solution was heated and stirred from 125° C. up to themaximum temperature of 140° C. for 30 minutes (step S15). Into thesolution was then added bismuth-2-ethylhexanate dissolved in 10 ml ofxylene (step S16), and the solution was heated and stirred from 130° C.up to the maximum temperature of 150° C. for 10 hours (step S17).

The solution was then distilled at a temperature of 130° C. to 150° C.for 5 hours so as to remove the low molecular-weight alcohol, water andxylene used as a solvent from the solution. The solution was thenfiltrated with a filter of 0.45 μm diameter so as to remove dusts fromthe solution (step S18). Subsequently, the concentration of SrBi₂ Ta₀.8Nb₁.2 O₉ in the solution was adjusted to 0.1 mol/l to produce theprecursor solution (b) (step S19).

The above precursor solutions (a) and (b) were used to form a thinferroelectric film having a three-layer laminated structure ofSBT/SBTN/SBT on the substrate including the lower Pt electrode 4.

Referring to FIG. 4, the precursor solution (a) was dropwise added ontothe lower Pt electrode 4 for spin coating at 3000 rpm for 20 seconds(step S20). The substrate was then mounted on a hot plate heated to 120°C. and was baked and dried at 120° C. in an air for 5 minutes (stepS21). Then, the wafer (substrate) was mounted on a hot plate heated to250° C. and was baked and annealed in an air for 5 minutes so as toallow the solvent to be completely evaporated, thereby completing theSBT layer 5a (the first layer) (step S22).

Subsequently, the precursor solution (b) was dropwise added onto the SBTlayer 5a (the first layer) for spin coating at 3000 rpm for 20 seconds(step S23). The substrate was then mounted on a hot plate heated to 120°C. and was baked and dried at 120° C. in an air for 5 minutes (stepS24). Then, the wafer (substrate) was mounted on a hot plate heated to250° C. and was baked and annealed in an ordinary air for 5 minutes soas to allow the solvent to be completely evaporated, thereby completingthe SBTN layer 5b (the second layer) (step S25).

Subsequently, the precursor solution (a) was dropwise added onto theSBTN layer 5b (the second layer) for spin coating at 3000 rpm for 20seconds (step S26). The substrate was then mounted on a hot plate heatedto 120° C. and was baked and dried at 120° C. in an air for 5 minutes(step S27). Then, the wafer (substrate) was mounted on a hot plateheated to 250° C. and was baked and annealed in an air for 5 minutes soas to allow the solvent to be completely evaporated, thereby completingthe SBT layer 5c (the third layer) (step S28).

A provisional annealing was then conducted in an oxygen atmosphere at580° C. for 30 minutes as the first annealing by employing RTA (RapidThermal Annealing) method (step S29), and an upper Pt electrode 6 wasdeposited to a thickness of 150 nm using a mask by EB (Electron Beam)deposition method to form an electrode of 100 μmφ (step S30).

A main annealing was then conducted in an oxygen atmosphere at 750° C.for 30 minutes as the second annealing by employing RTA method (stepS31).

The above steps completed the fabrication of the laminated thinferroelectric film 5 including SBT layer 5a/SBTN layer 5b/SBT layer 5ceach having a thickness of 67 nm and the total thickness being 200 nm(step S32).

The resistivity of the SBT film thus formed was 3.8×10¹³ Ω.cm and theresistivity of the SBTN film was 2.2×10¹¹ Ω.cm, so that the SBT filmshowed a larger resistivity than the SBTN film. The resistivity of thelaminated thin ferroelectric film including the SBT layer/SBTN layer/SBTlayer according to this embodiment was 3.3×10¹³ Ω.cm.

Comparison Example 1

A thin ferroelectric film element including a single SBT layer of 200 nmwas formed as the Comparison Example 1. The thin ferroelectric film wasmanufactured in the same manner (thermal processing temperature, elementstructure etc.) as in the Example 1 except that the steps of S20 to S22of FIG. 4 were repeated three times using only the precursor solution(a) and the steps from S29 onward were subsequently performed.

Comparison Example 2

A thin ferroelectric film element including a single SBTN layer of 200nm was formed as the Comparison Example 2. The thin ferroelectric filmwas manufactured in the same manner (thermal processing temperature,element structure etc.) as in the Example 1 except that the steps of S20to S22 of FIG. 4 were repeated three times using only the precursorsolution (b) and the steps from S29 onward were subsequently performed.

The ferroelectric properties of Example 1 and Comparison Examples 1 and2.

The ferroelectric properties were measured by use of a Sawyer Towercircuit shown in FIG. 5 by applying a voltage to capacitors of a typeshown in FIG. 1. The Sawyer Tower circuit shown in FIG. 5 is utilized bybeing connected to a measurement apparatus such as an oscilloscope fordisplaying. In the measurement of Example 1, a voltage V_(X) obtained bydividing the voltage V applied to the thin ferroelectric element isinputted into the terminal for the axis of abscissas of theoscilloscope. Assuming the polarization surface charge density of thethin ferroelectric film to be P and the true charge surface density tobe D, when a reference capacitor having a capacitance of C_(R) isconnected as shown in FIG. 5, the value (P+ε₀ E)×A, namely, D×A (whereinA is an area of the electrode) and the charge C_(R) V_(X) stored in thereference capacitor are both equal to Q, so that the voltage V_(Y)(D×A/C_(R)) which is proportional to D is inputted into the terminal forthe axis of ordinates of the oscilloscope.

Here, since P is sufficiently larger than E in ferroelectric substances,it can be assumed that D=P. When this V_(Y) -V_(X) curve is plottedagain using the film thickness, the voltage partition ratio, the area(A) of the electrode and the capacitance C_(R) of the referencecapacitor which are known values, a P-E (residual spontaneouspolarization-electric field) hysteresis curve or a D-E (stored electriccharge-electric field) hysteresis curve is obtained. From these curves,it is possible to read the residual spontaneous polarization (Pr), thecoercive field (Ec) and the stored electric charge (ΔQ).

FIGS. 6, 7 and 8 show the results obtained by measuring theferroelectric properties as the applied voltage is varied from 1 to 12 Vin the thin ferroelectric film elements of Example 1 and the ComparisonExamples 1 and 2 by employing the Sawyer Tower method. Referring toFIGS. 6, 7 and 8, the symbol  represents the Example 1 (the laminatedthin ferroelectric film of SBT/SBTN/SBT), the symbol ◯ represents theComparison Example 1 (the thin ferroelectric film of single SBT layer),and the symbol □ represents the Comparison Example 2 (the thinferroelectric film of single SBTN layer).

FIG. 6 is a view showing how the residual spontaneous polarization Prdepends on the applied voltage in Example 1, Comparison Example 1 andComparison Example 2. FIG. 6 shows that Example 1 of the presentinvention has a residual spontaneous polarization Pr which is about 1.7times larger (when a voltage of 3 V is applied) than that of ComparisonExample 1 (the thin ferroelectric film of single SBT layer) and isextremely advantageous in reading out the memory when the thinferroelectric film is used as a memory element. Further, the Example 1of the present invention shows a saturation property of Pr similar tothat of Comparison Example 1, namely, an excellent saturation propertyof the value Pr relative to the variation in the applied voltage,although the residual spontaneous polarization Pr is inferior to that ofComparison Example 2 including SrBi₂ Ta₀.8 Nb₁.2 O₉ obtained by addingNb into SrBi₂ Ta₂ O₉. From these, it is understood that the laminatedthin ferroelectric film of SBT layer/SBTN layer/SBT layer according toExample 1 of the present invention has both the excellent Pr saturationproperty of SBT and the high residual spontaneous polarization Pr ofSBTN.

FIG. 7 is a view showing how the coercive field Ec depends on theapplied voltage in Example 1, Comparison Example 1 and ComparisonExample 2. FIG. 7 shows that Example 1 according to the presentinvention has a sufficiently small coercive field Ec which is not sodifferent from that of Comparison Example 1 (the thin ferroelectric filmof single SBT layer), whereas the Comparison Example 2 including SrBi₂Ta₀.8 Nb₁.2 O₉ obtained by adding Nb into SrBi₂ Ta₂ O₉ shows a largecoercive field Ec. Further, the Example 1 according to the presentinvention shows a saturation property of Ec similar to that ofComparison Example 1 (the thin ferroelectric film of single SBT layer),namely, an excellent saturation property of the value Ec relative to thevariation in the applied voltage, although the coercive field Ec is alittle inferior to that of Comparison Example 1.

FIG. 8 is a view showing how the switching electric charge ΔQ depends onthe applied voltage in Example 1, Comparison Example 1 and ComparisonExample 2. FIG. 8 shows the same tendency as the dependence of theresidual spontaneous polarization Pr on the applied voltage shown inFIG. 6. In other words, the laminated thin ferroelectric film of SBTlayer/SBTN layer/SBT layer according to Example 1 of the presentinvention has both the excellent ΔQ saturation property of SBT relativeto the applied voltage and the high switching electric charge ΔQ ofSBTN.

It seems that the above-mentioned good properties of Example 1 have beenobtained by forming a laminated structure of a plurality of layersincluding a ferroelectric material having approximately the same crystalstructure so as to grow crystals without deteriorating thecharacteristics such as residual spontaneous polarization.

The fatigue characteristics of Example 1 and Comparison Examples 1 and2.

FIG. 9 is a view showing the measurement of the variation in the storedelectric charge relative to the repetition times when a voltage of 3 Vis applied. Referring to FIG. 9, the symbol  represents the Example 1(the laminated thin ferroelectric film of SBT/SBTN/SBT), the symbol ◯represents the Comparison Example 1 (the thin ferroelectric film ofsingle SBT layer), and the symbol □ represents the Comparison Example 2(the thin ferroelectric film of single SBTN layer).

When the stored electric charge of each case after 2×10¹¹ timesrepetition is compared in FIG. 9, it is understood that the storedelectric charges of Example 1 and Comparison Example 1 (the thinferroelectric film of single SBT layer) decrease little, whereas thestored electric charge of Comparison Example 2 (the thin ferroelectricfilm of single SBTN layer) decreases to 90%. Also, the stored electriccharge of Example 1 is itself significantly larger than that ofComparison Example 1 although it is a little inferior than that ofComparison Example 2. These show that Example 1 according to the presentinvention shows little fatigue associated with the polarizationinversion while maintaining the high stored electric charge of SBTN.

The leakage current characteristics of Example 1 and Comparison Examples1 and 2.

Ferroelectric memories have a non-volatile property of storing thememory contents even when the power is turned off. Therefore, when theferroelectric memories are applied to NVDRAMs which operate like DRAMsin an ordinary operation, the large leakage current causes problems suchas a decrease in refreshing time. On the other hand, if the leakagecurrent can be decreased by some orders of magnitude while maintainingthe stored electric charge to be constant, the refreshing time can beelongated, thereby greatly improving the memory device characteristics.Also, if the leakage current is large, the electric field applied to thethin ferroelectric film will be small, causing problems such asinsufficient polarization inversion. From these viewpoints, the leakagecurrent should preferably be as small as possible.

Table 1 shows the results of measuring the variation in the leakagecurrent densities together with the resistivities thereof in the thinferroelectric film element of Example 1 and the thin ferroelectric filmelements of Comparison Examples 1 and 2 when a voltage of 3 V isapplied.

                  TABLE 1                                                         ______________________________________                                        3V applied                                                                                  Leakage current                                                                         Resistivity                                                         (A/cm.sup.2)                                                                            (Ω cm)                                          ______________________________________                                        Embodiment 1    4.56 × 10.sup.-9                                                                    3.3 × 10.sup.13                             Comparison Example 1                                                                          3.95 × 10.sup.-9                                                                    3.8 × 10.sup.13                             Comparison Example 2                                                                          6.79 × 10.sup.-7                                                                    2.2 × 10.sup.11                             ______________________________________                                    

When Example 1 according to the present invention is compared withComparison Examples 1 and 2, it is understood that Example 1 provides aresistivity which is of the same orders of magnitude as ComparisonExample 1 and is higher by two orders of magnitude than ComparisonExample 2. Also, the leakage current of Example 1 is of the same ordersof magnitude as Comparison Example 1and is lower by two orders ofmagnitude than Comparison Example 2. These show that Example 1 has aleakage current lower by two orders of magnitude than SBTN whilemaintaining the high residual spontaneous polarization of SBTN byinserting the SBTN layer having the lower resistivity between the SBTlayers having the higher resistivity. Since the ferroelectric layerhaving a high resistivity provides an effect of shutting the leakagecurrent off, it seems that the leakage current in the thin ferroelectricfilm according to Example 1 of the present invention has become as smallas the leakage current of the ferroelectric layer having the higherresistivity by inserting the ferroelectric film having the lowerresistivity between the ferroelectric layers having the higherresistivity. In other words, it is possible to improve the leakagecurrent properties without deteriorating the ferroelectric properties byusing a ferroelectric layer having a high resistivity as shown inExample 1 of the present invention instead of using an ordinarydielectric layer having a high resistivity.

From the above-mentioned results, it has been found out that it ispossible to achieve a thin ferroelectric film element with low coercivefield Ec and low leakage current and with little fatigue whilemaintaining the high residual spontaneous polarization Pr and the highswitching electric charge ΔQ of SBTN by adopting a laminated thinferroelectric film of SBT layer/SBTN layer/SBT layer in the thinferroelectric film element according to Example 1 of the presentinvention.

Examples 2 to 6

Five kinds of thin ferroelectric film elements were fabricated bychanging the lamination pattern of SBT layers and SBTN layers used inExample 1, namely, by exchanging these layers and also varying thenumber of these layers to be laminated. The physical properties of theobtained five thin ferroelectric film elements were measured.

These elements were fabricated by merely changing the order of stepssimilar to those of Example 1 for applying the precursor solutions (a)and for applying the precursor solution (b) or by changing the number ofsteps to be performed. The other conditions for forming the film was thesame as those of Example 1. Also, the structures of these elements werethe same as those of Example 1 except that the order of the SBT layersand the SBTN layers to be laminated was changed.

FIGS. 10(a) to 10(e) show the film structures of Examples 2 to 6. Inaddition to these thin ferroelectric elements, FIGS. 10(f), 10(g) and10(h) show the film structures of the thin ferroelectric film elementsof Example 1, Comparison Example 1, and Comparison Example 2,respectively. Referring to FIGS. 10(a) to 10(h), the layer (A)represents a SrBi₂ Ta₂ O₉ layer (SBT layer) and the layer (B) representsa SrBi₂ Ta₀.8 Nb₁.2 O₉ layer (SBTN layer). The substrate is omitted inthe drawings.

Table 2 shows the results of measuring the residual polarization Pr, thecoercive field Ec, the switching electric charge δQ, and the leakagecurrent density I_(L) in these eight thin ferroelectric film elements.In Table 2, the measuring units are: μC/cm² for the residualpolarization Pr, kV/cm for the coercive field Ec, μC/cm² for theswitching electric charge δQ, and A/cm² for the leakage current densityI_(L).

                  TABLE 2                                                         ______________________________________                                                                  Switching                                                                              Leakage                                            Residual                                                                              Coercive  electric current                                            Polarization                                                                          field     charge   density                                             Pr!     Ec!       δ Q!                                                                             I.sub.L !                                 ______________________________________                                        Example 1f                                                                              6.96      34.5      12     1.28 × 10.sup.-8                    thin film ABA!                                                               Example 2a                                                                              7.75      44        14     9.64 × 10.sup.-8                    thin film BBA!                                                               Example 3c                                                                              6.55      46.1      12     9.26 × 10.sup.-9                    thin film ABB!                                                               Example 4d                                                                              6.13      36.4      10.5   1.37 × 10.sup.-8                    thin film AAB!                                                               Example 5e                                                                              8.62      43.2      15.7   9.57 × 10.sup.-8                    thin film BAB!                                                               Example 6b                                                                              8.18      28.9      14.1   3.45 × 10.sup.-8                    thin film BAA!                                                               Comparison                                                                              5.1       24.5       8.3   3.95 × 10.sup.-9                   Example 1g                                                                     thin film AAA!                                                               Comparison                                                                              8.7       55.1      16.0   6.79 × 10.sup.-7                   Example 2h                                                                     thin film BBB!                                                               ______________________________________                                    

Table 2 shows that the structures with an SBTN layer at a positionnearest to the lower electrode (thin films BBA, BAA, BAB, BBB) provide alarge residual polarization Pr. When these are compared with thestructures having an SBT layer at a position nearest to the lowerelectrode (thin films ABB, ABA, AAB, AAA), it is understood that thesestructures have approximately the same residual polarization Prirrespective of the number of SBTN layers except for the case of thethin film AAA having no SBTN layer. Therefore, it has been found outthat, in case of the thin films having an SBTN layer, the number of SBTNlayers has little influence on the residual polarization Pr.

On the other hand, the coercive field Ec tends to increase as the numberof SBTN layers increase. The leakage current I_(L) is as small as about10⁻⁹ to 10⁻⁸ except for the case of the thin film BBB.

From the result of measuring these properties in the Examples, it wasfound out that the film having the most excellent ferroelectric propertywas the thin film BAA which showed high Pr and low Ec, and the filmhaving the most inferior ferroelectric property was the thin film ABBwhich showed low Pr and high Ec. From these, it is understood that theresidual polarization Pr, the switching electric charge δQ and theleakage current density I_(L) do not depend so much on the number ofSBTN layers, but the properties depend on which of the SBTN layer andthe SBT layer is disposed at a position nearest to the lower electrode.Especially, it has been found out that good ferroelectric properties areobtained in the thin ferroelectric film element in which the SBTN layeris disposed at a position nearest to the lower electrode.

Also, good results similar to the above were observed when the fatigueproperty was measured in the thin ferroelectric film element having thethin film BAA in the same manner as in Example 1. Accordingly, it hasbeen found out that it is possible to improve the fatigue property whilemaintaining the high ferroelectric property by forming a laminatedstructure instead of forming a thin ferroelectric film of single SBTNlayer.

Example 7

A thin ferroelectric element having a laminated structure of SBT/Bi₄ Ti₃O₁₂ (hereafter referred to as "BTO")/SBT was formed instead of the thinferroelectric element having a laminated structure of SBT/SBTN/SBTaccording to the above Example 1, and the various properties thereofwere measured.

The element was fabricated by employing a sol-gel method in the samemanner as in the above Examples, using octylic acid solution of Sr, Biand Ti or octylic acid solution of Bi and Ti as a starting material forthe precursor solutions, and dispersing the octylic acid solution intoxylene for mixing so that the ratio of Sr/Bi/Ti will be 1/4/4 for theSBT layer and the ratio of Bi/Ti will be 4/3 for the BTO layer. Theconcentration and the viscosity of the solvent were adjusted to producethe precursor solutions, which were then applied onto a substrate byspin coating method.

Specifically, the precursor solution for the first SBT layer was firstapplied on the substrate at 5000 rpm for 20 seconds by spin coatingmethod, and the substrate was baked in an oven at 115° C. for 15 minutesin a drying step. A provisional annealing step was then conducted at400° C. for 60 minutes. The second BTO layer and the third SBT layerwere formed in the same manner as in the above process. Subsequently, amain annealing step was conducted in an oxygen atmosphere at 650° C. for15 seconds by RTA method to form the laminated thin ferroelectric filmof SBT layer/BTO layer/SBT layer. An upper electrode was formed on thelaminated thin ferroelectric film thus formed in the same manner as inthe above Example 1 to complete the thin ferroelectric film elementaccording to this Example.

Comparison Example 3

A thin ferroelectric film element having a thin ferroelectric film ofsingle BTO layer was fabricated in the same manner as in ComparisonExample 1.

The result of Example 7 and Comparison Example 3

Table 3 shows the results of measuring the residual polarization Pr, thecoercive field Ec, the leakage current and the resistivity in the thinferroelectric film elements of Example 7 (laminated thin ferroelectricfilm of SBT layer/BTO layer/SBT layer), Comparison Example 1 (thinferroelectric film of single SBT layer) and Comparison Example 3 (thinferroelectric film of single BTO layer), in the same manner as in theabove Examples.

                  TABLE 3                                                         ______________________________________                                                                Leakage                                                      Pr       Ec      current     Resistivity                                      (μC/cm.sup.2)                                                                       (kV/cm) (A/cm.sup.2)                                                                              (Ω cm)                              ______________________________________                                        Embodiment 7                                                                           8.6        103     7.89 × 10.sup.-8                                                                  1.9 × 10.sup.12                   Comparison                                                                             4           92     6.85 × 10.sup.-8                                                                  2.2 × 10.sup.12                   Example 1                                                                     Comparison                                                                             11.6       111     5.37 × 10.sup.-6                                                                  2.8 × 10.sup.10                   Example 3                                                                     ______________________________________                                    

Table 3 shows that the residual polarization Pr according to Example 7is two times as large as that of Comparison Example 1. Theabove-mentioned good property of Example 7 has been obtained by forming,as in the above-described Examples, a laminated structure of a pluralityof thin ferroelectric films including a SBT ferroelectric layer havingapproximately the same crystal structure so as to grow crystals withoutdeteriorating the characteristics such as residual polarization.

Moreover, in Comparison Example 3 (single BTO layer), the leakagecurrent is large and the resistivity is small. In Comparison Example 1(single SBT layer), the leakage current is smaller by two orders ofmagnitude and the resistivity is larger by two orders of magnitude, thusshowing good values. On the other hand, Example 7 according to thepresent invention shows a leakage current which is smaller by two ordersof magnitude than that of Comparison Example 3 while maintaining thehigh residual polarization Pr of BTO.

Since the ferroelectric layer having a high resistivity provides aneffect of shutting the leakage current off, it seems that the leakagecurrent in the thin ferroelectric film according to Example 7 of thepresent invention has become as small as the leakage current of theferroelectric layer having the higher resistivity by inserting theferroelectric film having the lower resistivity between theferroelectric layers having the higher resistivity, as in the aboveExample 1. In other words, it is possible to improve the leakage currentproperties with little deterioration in the ferroelectric properties byusing a ferroelectric layer having a high resistivity as shown inExample 7 of the present invention instead of using an ordinarydielectric layer having a high resistivity.

Example 8

The thin ferroelectric film fabricated according to Example 1 of thepresent invention was applied to a non-volatile memory having acapacitor structure.

FIG. 11 is a schematic view showing an essential part of a non-volatilememory having a capacitor structure according to Example 8 of thepresent invention. Referring to FIG. 11, the non-volatile memorycomprises memory cells and each of the memory cells includes a capacitor24 and a transistor 23. The capacitor 24 has a structure such that thelaminated thin ferroelectric film 5 fabricated according to the presentinvention is interposed between a pair of electrodes (conductors) 26 and26'. The transistor 23 includes a bit line 28, a word line 27 and asignal line 29 connected to an Al electrode 25. The Al electrode 25 isalso connected to the electrode 26' of the capacitor 24.

Next, the method for fabricating the non-volatile memory according toExample 1 of the present invention will be explained. First, SiO₂ andSi₃ N₄ are formed on an n-type silicon substrate. Field oxidation isthen performed by photoetching to form a field SiO₂, leaving the Si₃ N₄at a portion where a transistor is to be formed later. Subsequently, thepreviously formed Si₃ N₄ film and the SiO₂ film immediately thereunderare removed. Then, after a gate SiO₂ is formed with a gate oxide film, apolysilicon gate 27 is formed. By using the gate 27 as a mask, ionimplantation is carried out to form a source 28 and a drain 29. Thesurface is then covered with PSG (Phospho-Silicate Glass), followed byreflowing to planarize the surface. Then, after an electrode 26 isformed thereon, a laminated thin ferroelectric film 5 is formed on theelectrode 26 in the same manner as in the above-described Example 1, andan electrode 26' is formed thereon. Subsequently, the surface is coveredwith PSG again, followed by reflowing to planarize the surface. Acontact hole is then formed on the electrode 26' and on the drain 29 byetching. Finally, an Al electrode 25 is formed for wiring.

Next, the operation of the non-volatile memory having a capacitorstructure according to this Example of the present invention will beexplained by referring to FIG. 12 which shows an equivalent circuitthereof. For writing "1" into the memory, a negative pulse larger thanthe coercive field of the laminated thin ferroelectric film 5 is appliedto the film 5 from the bit line 28 via the transistor 23 so as togenerate dielectric polarization in the laminated thin ferroelectricfilm 5, thereby storing negative residual polarization charge in theelectrode 26 of the capacitor 24 for writing. For writing "0" into thememory, a positive pulse larger than the coercive field is applied tothe laminated thin ferroelectric film 5 from the bit line 28 via thetransistor 23 so as to generate dielectric polarization in the laminatedthin ferroelectric film 5, thereby storing positive residualpolarization charge in the electrode 26 of the capacitor 24 for writing.

For reading "1" out of the memory, a positive pulse is applied, wherebypolarization inversion takes place to store positive residualpolarization charge in the electrode 26 of the capacitor 24 instead ofthe previous negative residual polarization charge. This causes a changein the amount of stored electric charge, the change being equal to thedifference between the negative residual polarization charge before theapplication of the pulse and the positive residual polarization chargeafter the application of the pulse, making it possible to read "1" outof the memory. On the other hand, for reading "0" out of the memory, apositive pulse is applied, whereby no polarization inversion takesplace, causing little change in the amount of stored electric chargebefore and after the application of the pulse, making it possible toread "0" out of the memory. In practice, the reading may be conducted bysensing the difference in the amount of stored electric charge beforeand after the application of the pulse with a sensing amplifier or thelike connected to the bit line to identify the difference as bitinformation. Since the laminated thin ferroelectric film 5 has residualpolarization, the state of "1" or "0" is maintained even after the poweris turned off, thereby realizing the non-volatile memory operation.

Here, the memory having a similar capacitor structure can be operated asa DRAM by utilizing only the high dielectric property of theferroelectric substance. In this case, the memory can be operated as anon-volatile memory only when the power is off.

Example 9

By use of FIG. 13, an explanation will be given on a case in which thethin ferroelectric film according to Example 1 of the present inventionis applied to a MFMIS-FET (Metal Ferroelectric Metal InsulatorSemiconductor FET).

FIG. 13 is a schematic view showing an essential part of MFMIS-FET.Referring to FIG. 13, the element includes, on an n-type siliconsubstrate surface 1, a drain region 35 and a source region 36, on whichare formed a SiO₂ film (gate insulating film) 30, a floating gate 31, alaminated thin ferroelectric film 32, a control gate 33 and a wiring 34,successively in this order.

The method for manufacturing the element according to Example 9 will beexplained. First, as in Example 8, a gate insulating film (SiO₂) 30 isformed on an n-type silicon substrate 1. After a floating gate 31 isformed thereon with Pt, a drain 35 and a source 36 are formed by ionimplantation. The surface is then covered with PSG (Phospho-SilicateGlass), followed by reflowing to planarize the surface. Subsequently,the PSG on the floating gate 31 by Pt is removed by etching and alaminated thin ferroelectric film 32 is formed on the floating gate 31as in Example 1. Further, a control gate 33 is formed thereon with Pt.The surface is then covered with PSG again, followed by reflowing toplanarize the surface. A contact hole is then formed on the control gate33, the drain 35, and the source 36 by etching. Finally, an Al wiring 34is formed.

Next, the operation of the MFMIS-FET according to Example 9 will beexplained. In the MFMIS-FET, a voltage is applied to the control gate 33so as to change the polarization direction of the laminated thinferroelectric film 32, whose electrostatic induction causes dielectricpolarization to change in the gate insulating film 30 (SiO₂ film) viathe floating gate 31. The direction of the dielectric polarizationcontrols the formation of channel regions in the substrate surfaceimmediately under the floating gate. This allows determination of thevalues "1" and "0" by ON-OFF of the drain current.

Suppose, for example, that the control gate 33 is in a state of zerobias, if the laminated thin ferroelectric film 32 is polarized towardsthe semiconductor substrate (the silicon substrate 1) to allow thefloating gate 31 side of the laminated thin ferroelectric film 32 tohave a negative polarity, the SiO₂ film 30 will be dielectricallypolarized to allow the surface of the SiO₂ film 30 contacting the Sisubstrate 1 to have a negative polarity, whereby the surface of the Sisubstrate 1 contacting the SiO₂ film 30 will have a positive polarity,thus electrically disconnecting the drain 35 and the source 36 (the OFFstate).

Next, when a positive voltage larger than the coercive field of thelaminated thin ferroelectric film 32 is applied to the control gate 31,the polarization of the laminated thin ferroelectric film 32 will bereversed so that the floating gate 31 side of the laminated thinferroelectric film 32 will have a positive polarity. In this case, theSiO₂ film 30 will be dielectrically polarized to allow the surface ofthe SiO₂ film 30 contacting the Si substrate 1 to have a positivepolarity, whereby the surface of the Si substrate 1 contacting the SiO₂film 30 will have a negative polarity, thus electrically connecting thedrain 35 and the source 36 (the ON state).

When the control gate 31 is allowed to have a voltage of zero bias inthis state (the ON state), the residual polarization of the laminatedthin ferroelectric film 32 will keep this state unchanged. At thisstate, since the dielectric polarization of the SiO₂ film 30 is keptunchanged as long as the polarization of the laminated thinferroelectric film 32 is maintained, the MFMIS-FET can be operated as anon-volatile memory from which data can be non-destructively read out.

According to the thin ferroelectric film element of the presentinvention thus described, it is possible to manufacture an element whichcan achieve sufficiently high spontaneous residual polarization andsufficiently low coercive field with a lower annealing temperature thanby the conventional methods and in which the operating voltage is small,the leakage current is small, and the fatigue characteristics areexcellent.

More specifically described, although the residual polarization of SrBi₂Ta₀.8 Nb₁.2 O₉ (SBTN) is 1.7 times larger than that of SrBi₂ Ta₂ O₉(SBT), the coercive field and the leakage current were large and hencehad to be largely improved. However, as shown in the above embodimentsof the present invention, it is possible to largely improve the coercivefield and the leakage current density, while substantially maintainingthe high residual polarization and the switching electric charge ofSBTN, by providing a laminated structure of SrBi₂ Ta₀.8 Nb₁.2 O₉ (SBTN)which is a low resistance layer and SrBi₂ Ta₂ O₉ (SBT) which is a highresistance layer. This leads to a great advantage in reading out thememory when the thin ferroelectric film is furnished in an electronicdevice.

Also, by providing a laminated structure of SBTN and SBT, the coercivefield of the thin ferroelectric film according to the present inventioncan be reduced to be as small as about 50% of the coercive field of SBTNand will have a good saturation property relative to the applied voltagein which the coercive field is saturated at about 3V, whilesubstantially maintaining the high residual polarization of SBTN. Thiswill be effective in reducing the operation voltage when the thinferroelectric film is furnished in an electronic device.

Also, by providing a laminated structure of SBTN and SBT, the leakagecurrent density of the thin ferroelectric film according to the presentinvention can be reduced by two orders of magnitude compared with theleakage current density of SBTN, while substantially maintaining thehigh residual polarization of SBTN. This will elongate the period oftime required for refreshing when the memory is operated as a DRAM.Further, if the memory is operated as a FRAM, sufficient voltage will beapplied to the thin ferroelectric film to ensure sufficient polarizationinversion, thereby improving the stability and the reliability of theoperation.

Moreover, while the switching electric charge decreased to about 90% ofthe original amount after 2×10¹¹ times repetition of the polarizationinversion in a single SBTN layer, there will be little deterioration inthe fatigue property of the thin ferroelectric film according to thepresent invention by providing a laminated structure of SBTN and SBT,thereby greatly reducing the fatigue accompanying the repetition of thepolarization inversion.

Further, not only the laminated structure of SBTN and SBT but also alaminated structure of SBT (SrBi₂ Ta₂ O₉) and Bi₄ Ti₃ O₁₂ or the likestructure can achieve a similar improvement in these characteristics.

What we claim is:
 1. A thin ferroelectric film element comprising upperand lower thin electrode films and a thin ferroelectric film formed on asubstrate, wherein the thin ferroelectric film comprises at least threeferroelectric layers in which (1) at least one layer has a compositionof constituent elements different from at least two other layers and aresistivity higher than that of at least two other layers; and (2) atleast two of the other layers have the same composition of constituentelements.
 2. The thin ferroelectric film element according to claim 1,wherein at least one layer of the thin ferroelectric film comprises abismuth layered-structure compound material.
 3. The thin ferroelectricfilm element according to claim 2, wherein each of the at least threelayers of the thin ferroelectric film contains at least one metalelement selected from a group consisting of Sr, Bi, Ti, Ta and Nb. 4.The thin ferroelectric film element according to claim 1, wherein the atleast one layer having a resistivity higher than that of at least twoother layers has a resistivity of more than 10¹² Ω.cm.
 5. The thinferroelectric film element according to claim 1, wherein each of the atleast three layers of the thin ferroelectric film has the thicknesswithin a range of from 100 Å to 1000 Å.
 6. The thin ferroelectric filmelement according to claim 1, which is used to form a circuit portion ofan integrated circuit constituting a semiconductor device.
 7. A methodof manufacturing a thin ferroelectric film element of claim 1comprising:forming a lower thin electrode film on a substrate; applyingeach of a plurality of precursor solutions containing partiallydifferent metal elements and drying to form a laminate film of at leastthree layers comprising at least two kinds of films; performing a firstannealing; forming an upper thin electrode film; and performing a secondannealing.
 8. The thin ferroelectric film element according to claim 1,wherein each of the at least three layers of the thin ferroelectric filmis formed from a member selected from a group consisting of SrBi₂ Ta₂O₉, SrBi₂ Ta₀.8 Nb₁.2 O₉, Bi₄ Ti₃ O₁₂, SrBi₄ Ti₄ O₁₅, SrBi₄ (Ti, Zr)₄O₁₅, SrBi₂ Nb₂ O₉, CaBi₂ Ta₂ O₉, BaBi₂ Ta₂ O₉, BaBi₂ Nb₂ O₉, and PbBi₂Ta₂ O₉.
 9. The thin ferroelectric film element according to claim 1,wherein the at least one layer having a resistivity higher than that ofat least two other layers has a resistivity within a range of about 10¹³to about 10¹² Ω.cm.